Currently available robot arm mechanisms include pivotally joined multiple links that are driven by a first motor and are mechanically coupled to effect straight line movement of an end effector or hand and are equipped with a second motor to angularly displace the hand about a central axis. Certain robot arm mechanisms are equipped with telescoping mechanisms that move the hand also in a direction perpendicular to the plane of straight line movement and angular displacement of the hand. The hand is typically provided with a vacuum outlet or some other gripping mechanism that secures a specimen, such as a semiconductor wafer, computer hard disk, or compact disk, to the hand as it transports the specimen between processing stations.
U.S. Pat. No. 4,897,015 of Abbe et al. describes a rotary-to-linear motion robot arm that uses a first motor to control a multiple link robot arm to produce straight line radial motion from motor-driven rotary motion. An additional motor may be coupled to the robot arm for operation independent of that of the first motor to angularly move the multiple link robot arm without radial motion. Because they independently produce radial motion and angular motion, the first and second motors produce useful robot arm movement when either one of them is operating.
The robot arm of the Abbe et al. patent extends and retracts an end effector (or hand) along a straight line path by means of a mechanism that pivotally couples in a fixed relationship a first arm (or forearm) and a second (or upper) arm so that they move in predetermined directions in response to rotation of the upper arm. To achieve angular displacement of the hand, a third drive motor rotates the entire robot arm structure. The Abbe et al. patent describes no capability of the robot arm to reach around corners or travel along any path other than a straight line or a circular segment defined by a fixed radius.
U.S. Pat. No. 5,007,784 of Genov et al. describes a robot arm with an end effector structure that has two oppositely extending hands, each of which is capable of picking up and transporting a specimen. The end effector structure has a central portion that is centrally pivotally mounted about the distal end of a second link or forearm. The extent of pivotal movement about all pivot axes is purposefully limited to prevent damage to vacuum pressure flexible conduits resulting from kinking or twisting caused by over-rotation in a single direction.
The coupling mechanism of a first link or upper arm, the forearm, and the end effector structure of the robot arm of the Genov et al. patent is more complex than that of the robot arm of the Abbe et al. patent. Nevertheless, the robot arm structures of the Abbe et al. and Genov et al. patents operate similarly in that each of the end effector structures picks up and transports specimens by using one motor to extend and retract a hand and another, different motor to rotate the entire robot arm structure to allow the hand to extend and retract at different ones of a restricted number of angular positions.
More complex movement capability is described in U.S. Pat. No. 5,765,444 for DUAL END EFFECTOR, MULTIPLE LINK ROBOT ARM SYSTEM WITH CORNER REACIIAROUND AND EXTENDED REACH CAPABILITIES, which is assigned to the assignee of this application and is incorporated herein by reference. A multiple link robot arm mechanism includes two coaxially arranged motors mounted that are capable of synchronized operation that moves a robot arm hand along a curvilinear path as the extension of the hand changes. The first motor rotates a forearm about an elbow axis that extends through distal and proximal ends ol the upper arm and forearm, respectively, and the second motor rotates an upper arm about a shoulder axis that extends through a proximal end of the upper arm. A mechanical linkage couples the upper arm and the forearm. The mechanical linkage forms an active drive link and a passive drive link. The active drive link operatively connects the first motor and the forearm to cause the forearm to rotate about the elbow axis in response to the first motor. The passive drive link operatively connects the forearm and the hand to cause the hand to rotate about a wrist axis in response to rotation of the forearm about the elbow axis. The wrist axis extends through distal and proximal ends of the forearm and hand, respectively.
Whenever the first and second motors move equal angular distances, the angular displacement of the upper arm about the shoulder axis and the angular displacement of the forearm about the elbow axis equally offset and thereby result in only a net angular displacement of the hand about the shoulder axis. Thus, under these conditions, there is no linear displacement of the hand and no rotation of the hand about the wrist axis. Whenever the first and second motors move different angular distances, the angular displacement of the upper arm about the shoulder axis and the angular displacement of the forearm about the elbow axis only partly offset and thereby result in angular displacements of the hand about the shoulder and wrist axes and a linear displacement of the hand. Accordingly, coordination of the position control of the first and second motors enables the robot arm mechanism to describe a compound curvilinear path of travel for the hand.
Such robot arm mechanisms are commonly employed to transport semiconductor wafers among storage cassettes, prealigners, and processing stations. Prior multiple link robot arm mechanisms employed a 2:1:1:2 drive ratio and were capable of moving 200 millimeter diameter wafers. However, semiconductor wafers are increasing in size and weight, making them more difficult to move rapidly and accurately over the extended distances required for 300 millimeter diameter wafers. Accordingly, additional positioning torque and extended reach capabilities were developed by the assignee of this application and described in U.S. patent application Ser. No. 09/098,167, filed Jun. 16, 1998 for SINGLE AND DUAL END EFFECTOR, MULTIPLE LINK ROBOT ARM SYSTEMS HAVING TRIAXIAL DRIVE MOTORS, which is assigned to the assignee of this application and is incorporated herein by reference. The robot arm described therein has a 1:1:1:2 drive ratio, which is advantageous because it has twice the angular drive resolution and transmits twice the motor torque to the robot arm mechanism, thereby allowing longer links for an extended reach capability.
However, the 1:1:1:2 drive ratio has a disadvantage that has discouraged its use. Each rotation of the first motor relative to the second motor causes two extension and retraction cycles of the hand. Therefore, when the robot arm system is first powered up, it is impossible to determine whether a particular relative directional rotation of the first and second motors will extend or retract the hand, and in which of two opposite directions the hand will point.
What is needed, therefore, is a robot arm system that has straight line motion, extended reach, and corner reacharound capabilities for rapidly and accurately transporting large specimens to virtually any location in an available work space.